555 EFI again...

[Chris Morriss]

In message <35A14466.3AD1CF47 at brokersys.com>, Walter Petermann
<corsaro at brokersys.com> writes
>Frederic Breitwieser wrote:
>> 
>> >I'm finished with the circuit and am working on the power supply.
>> 
>> I would love a copy :)
>> 
>
>Here it is:
>
>http://brokersys.com/~corsaro/wiring_diagrams/555fe.htm
>
>let me know what you think.
>
> Walter
I'm writing this before looking at this website, so perhaps this design
is OK.  The point I want to make is that the core of a simple EFI is the
voltage to pulse-width stage.  If you use a 555 and simply alter the
threshold level you won't get a linear v to PW relationship.  A normal
555 monostable uses an R-C network for the timing.  The voltage across
the capacitor rises in an exponential fashion, whereas we require that
the voltage rises in a linear ramp.  This can be acheived by using a
constant current source to charge the cap.  Voltage across the cap will
vary according to dv/dt = i/C.

With this technique the desired pulse width can be set by altering the
threshold voltage, or by keeping a fixed threshold and making the
current source into a voltage controlled one.  This can be done by the
conventional op-amp and transistor VCCS, (ground referenced) with a
current mirror used to provide the current from the V+ rail.
-- 
Chris Morriss